1. Field of the Invention
The present invention relates to a liquid crystal display that can eliminate viewing angle dependence.
2. Description of the Related Art
A liquid crystal display (LCD) includes a pair of substrates and a liquid crystal layer (liquid crystal cell) sandwiched between them, and produces a display by altering the orientation of the liquid crystal molecules in the liquid crystal layer and thereby changing the optical refractive index within the liquid crystal cell. Accordingly, the liquid crystal molecules need to be aligned in an orderly manner within the liquid crystal cell.
One method commonly used to align the liquid crystal molecules in a given direction involves forming an alignment film on the liquid crystal layer side of each substrate and controlling the substrate surface condition in such a manner as to interact with the liquid crystal molecules. According to this method, a liquid crystal alignment film material is first applied on the facing surfaces of the pair of substrates, and then dried and cured to form an alignment film on each surface, and preferential orientation is given by rubbing the surface of the alignment film with a nylon cloth or the like (rubbing method).
An inorganic alignment film or an organic alignment film may be used as the alignment film for the above purpose. Oxides, organic silanes, metals, and metallic complexes are examples of the inorganic alignment film materials. As the organic alignment film materials, polyimide resins are widely used; by rubbing the polyimide film surface formed on the substrate, the liquid crystal molecules can be aligned in a given direction.
Of such liquid crystal displays, thin-film transistor (TFT) liquid crystal displays (TFT-LCDs) are constructed using twisted nematic liquid crystals. In the TN liquid crystal display, the liquid crystal molecules are arranged with their long axes lying substantially parallel to the pair of substrates and gradually twisting through 90.degree. between them; when a voltage is applied between electrode conductive lines formed on the respective substrates and an electric field is formed in a direction perpendicular to the substrates, the molecular alignment is altered with the liquid crystal molecules being caused to line up in the direction of the electric field by virtue of the dielectric anisotropy of the liquid crystal, thus producing a change on the optical refractive index within the liquid crystal layer.
In such a TN liquid crystal display, since the liquid crystal molecules have the property of refractive index anisotropy (birefringence), a phenomenon occurs in which the contrast varies depending on the angle at which the observer views the screen of the liquid crystal display. This phenomenon will be explained with reference to FIGS. 1, 2, and 3.
FIGS. 1 and 2 are a plan view and a perspective view, respectively, of a typical TN liquid crystal display, and FIG. 3 shows a cross section taken along line F-F' in FIG. 1. The liquid crystal display is an active matrix display, and includes a pair of wiring substrates 131 and 132 and a liquid crystal layer 133 sandwiched between them. One wiring substrate 131 includes a glass substrate 111a, a transparent pixel electrode 114, and an alignment film 116a, while the other wiring substrate 132 includes a glass substrate 111b, a transparent counter-electrode 115, and an alignment film 116b.
The edges of the two wiring substrates 131 and 132 are sealed with a resin or the like (not shown) in such a manner as to surround the liquid crystal layer 133. Peripheral circuits for driving the liquid crystal layer 133, etc. are mounted outward of the sealing resin. Around the pixel electrode 114 are arranged scanning lines 112 and signal lines 113 intersecting with each other. Electrical signals are applied to the scanning line 112 and signal line 113 connected to the pixel electrode 114 to drive the liquid crystal layer through a TFT 120.
Liquid crystal molecules 133a in the liquid crystal layer 133 placed between the two wiring substrates 131 and 132 are oriented in such a manner that they twist through 90.degree. between the two substrates 131 and 132, the average orienting direction of the liquid crystal molecules projected on the substrate being substantially parallel to the direction of line F-F'. Also, the liquid crystal molecules 133a have a pretilt angle .delta. with respect to the substrates 131 and 132. This pretilt angle .delta. is provided to prevent the occurrence of disclination lines due to multidomain; because of the pretilt angle .delta., when a voltage is applied between the pixel electrode 114 and the counter electrode 115, the liquid crystal molecules 133a line up uniformly in the direction of the pretilt angle .delta.. In FIG. 2, the arrow 134 indicates the rubbing direction of the substrate 131 and the arrow 135 the rubbing direction of the substrate 132, while the arrow 136 indicates the positive viewing direction. Such an arrangement is also employed in liquid crystal displays of other types than the active matrix.
In conventional liquid crystal displays, however, since the direction in which the liquid crystal molecules line up when an electric field is applied is predetermined, a phenomenon occurs in which the contrast varies depending on the angle at which the observer views the liquid crystal display. The reason why this phenomenon occurs will be explained with reference to FIG. 4 showing the voltage-transmittance (V-T) characteristics of a normally white mode liquid crystal display which produces a white display when no voltage is applied. Here, when the liquid crystal molecules 133a are viewed from the .theta.1 side in FIG. 3, the viewing direction is said to be the positive viewing direction, and when viewed from the .theta.2 side, it is said to be the negative viewing direction.
When the liquid crystal display is viewed from directly above (from a direction perpendicular to the substrate plane), a V-T characteristic such as shown by solid line L1 in FIG. 4 is obtained. As can be seen, as the applied voltage value increases, the light transmittance decreases until it becomes substantially zero at a certain applied voltage value, at voltages above which the transmittance remains substantially zero.
On the other hand, the viewing angle is shifted from the direct-above position toward the positive viewing direction (.theta.1 side in FIG. 3), a V-T characteristic such as shown by solid line L2 in FIG. 4 is obtained. As can be seen, the light transmittance decreases with increasing applied voltage until the voltage reaches a particular value, from which point the transmittance begins to increase and then gradually decreases. This means that at a particular angle of light incidence (viewing angle) the liquid crystal molecules are tilted in the same direction and the refractive index anisotropy of the liquid crystal molecules is lost, resulting in the loss of the optical rotatory power. That is, at a particular viewing angle an inversion phenomenon (contrast reversal) occurs in which the dark and light parts of an image appear as light and dark, respectively.
Conversely, when the viewing angle is shifted toward the negative viewing direction (.theta.2 side in FIG. 3), the refractive index of the liquid crystal molecules becomes difficult to change and the V-T characteristic shown by solid line L3 in FIG. 4 is obtained, which indicates that the light transmittance is hard to change. As a result, contrast between black and white drops markedly.
More specifically, when the applied voltage is zero or relatively low, the center molecule 133a appears as an ellipse to the observer 137 positioned in the positive viewing direction, as shown in FIG. 5A. When the applied voltage is gradually increased, the center molecule 133a tilts toward the direction of the electric field and there is an instant in time at which the center molecule 133a appears as a true circle to the observer 137, as shown in FIG. 5B. At this time, the light transmittance is the highest. When the applied voltage is further increased, the center molecule 133a stands up substantially parallel to the direction of the electric field, as shown in FIG. 5C, and again appears as an ellipse to the observer 137. In this manner, the refractive index (.DELTA. n) varies with the tilt angle of the liquid crystal molecule; therefore, as the viewing angle is shifted toward the positive viewing direction .theta.1, the inversion phenomenon in which the dark and light parts of an image appears reversed occurs at a particular angle.
In other viewing directions (the negative viewing direction) than the positive viewing direction .theta.1, no inversion phenomenon occurs because the V-T characteristic is different, but for the same reason as described above, there occurs a phenomenon in which the contrast ratio between black and white decreases with an increasing viewing angle.
In the TN liquid crystal display, the inversion phenomenon and decreased contrast as described above are very annoying to the observer, and make one doubt the display characteristics of the liquid crystal display.
Various methods have heretofore been proposed to improve the viewing angle characteristic peculiar to such TN liquid crystal displays and enhance the display quality. For example, Japanese Laid-open Patent Publication No. 60-211424 and The Institute of Electronics, Information and Communication Engineers, Technical Research Report ("Complementary TN (CTN)--TN with wide viewing angle--," pp. 35-41, February 1993) disclose methods in which each pixel is divided so that two or more different molecular orientations are provided. Furthermore, Japanese Laid-open Patent Publication No. 3-230120 ("Liquid Crystal Display " by Sharp Kabushiki Kaisha) proposes a method that uses a compensation plate, while Japanese Laid-open Patent Publication No. 1-200329 discloses a method for improving the viewing angle characteristic by adjusting the liquid crystal materials and liquid crystal cell thickness.
The various methods proposed for improving the viewing angle characteristic of the liquid crystal display, however, have had the following problems.
For example, the methods involving dividing every pixel and providing two or more different molecular orientations include, for example, a method in which an alignment film formed from an organic film is etched, or selectively masked by photolithography, and then subjected to rubbing so that a masked region is made a nonoriented region and an unmasked region an oriented region, thus forming differently oriented regions, especially regions with opposite orientations, within the same pixel area. According to this technique, each pixel can be formed with orientations for both the positive and negative viewing directions, so that the contrast decrease in the negative viewing direction can be prevented. However, in either method, foreign matter adheres to the alignment film or the alignment film is scratched, which may degrade the display quality of the liquid crystal display.
On the other hand, with the method involving the use of a compensation plate, the viewing angle cannot be increased on the opposite side from the side for which the compensation plate is intended, while with the method involving adjusting the liquid crystal materials and liquid crystal cell thickness, it is difficult to improve the quality of the liquid crystal display since the materials that can be used are limited.
Another known method for preventing the inversion phenomenon and contrast reduction is one disclosed in Japanese Laid-open Patent Publication No. 2-12. According to this method, which is used for an active matrix liquid crystal display, a display electrode forming each pixel is split into several parts and a capacitor is coupled to each split display electrode, making it possible to create orientations in several different directions by forming different electric fields within the same pixel, thus achieving an improvement in the viewing angle characteristic. While this method of driving the split display electrodes is effective in improving the viewing angle characteristic resulting from changes in retardation such as observed on normally black mode liquid crystal displays, little effect can be obtained in preventing the half-tone (gray-scale) inversion phenomenon caused by the tilting of liquid crystal molecules. That is, this method is effective in improving the viewing angle characteristic for normally black mode liquid crystal displays, but is not effective for normally white mode liquid crystal displays that provide good contrast.
Furthermore, all of the above-described methods have had the problem of requiring extra steps in the manufacture of liquid crystal displays, leading to increased manufacturing cost.